The present invention relates to white liquor utilized in the pulping of wood. Even more particularly, the present invention relates to a method of producing oxidized white liquor in which sodium sulfide contained within the white liquor is oxidized to sodium sulfate.
An initial stage in the production of wood pulp for paper making is the delignification of wood chips by the use of reprocessed white liquor. White liquor is typically an aqueous solution of sodium hydroxide (76 g/l), sodium carbonate (19 g/l), sodium sulfide (33 g/l) and sodium sulfate (2 g/l). The foregoing concentrations are exemplary only and each component could be more or less than that stated hereinbefore. The delignification creates black liquor which is concentrated in an evaporator. After concentration, the black liquor is burned in a furnace to produce an inorganic residue, known in the art as smelt. The smelt is dissolved in water to produce green liquor which is further processed in causticizing and clarifying stages to produce the white liquor. The white liquor is recycled back to the initial cooking stage. Some mills use oxidized white liquor (thiosulfate) for O.sub.2 delignification.
The successive pulp bleaching stages can consist of oxygen delignification, chlorine dioxide, oxidative extraction, with or without hydrogen peroxide or separate peroxide stages. Peroxide in oxidative extraction stages is consumed by the sodium thiosulfate present in conventionally processed white liquor should the liquor be used as a source of alkali. Hydrogen peroxide is expensive and its depletion adds an unnecessary cost burden to the bleaching process.
It is known that it would be very advantageous to render the white liquor inert to expensive oxidizing agents such as peroxide by oxidation of the sodium sulfide. Thereafter the oxidized white liquor could be utilized within alkaline oxidizing bleaching stages. The use of such oxidized white liquor would make it possible not only to economically improve the pulp production process through a reduction of the consumption of peroxide but also, to improve the product quality of the pulp. To this end, oxidized white liquor has been produced in which sodium sulfide is oxidized to sodium thiosulfate. Further oxidation would of course render the sodium sulfide inert to the action of powerful oxidants such as hydrogen peroxide and chlorine dioxide, but the oxidation of sodium sulfide to sodium sulfate has proved to be impractical due to slow reaction rates.
As will be discussed, the present invention provides a method of producing oxidized white liquor by oxidizing the sodium sulfide in the white liquor to sodium sulfate at a sufficiently rapid reaction rate so as to make the use of sodium sulfate containing white liquor industrially practical.